1. Field of the Invention
The present invention relates to an electric power-assist steering system for a vehicle, having an idle stop mechanism that automatically performs an idle stop operation under a predetermined condition.
2. Description of the Related Art
Recently, there have been produced more vehicles that perform an idle stop operation so as to reduce CO2 emission and enhance environmental conservation. In every idle stop operation, such a vehicle not only stops the engine, but also reduces power consumption of an electrically powered equipment so as to reduce load on the battery.
In addition, recently almost every vehicle employs an electric power-assist steering system. Such an electric power-assist steering system generates an assist force based on the steering torque and the steering angle, so as to enhance the steering performance of the vehicle. A steering angle sensor that detects a relative angle of a steering angle is used as a steering angle sensor for detecting such a steering angle (see JP 3728991B, for example). Such a steering angle sensor learns a midpoint of the steering angle, and compares a detected steering angle with the midpoint of the steering angle, thereby to obtain an actual steering angle of the steering wheel. In addition, recently more vehicles employ a VSA (Vehicle Stability Assist) system. Such a VSA system provides a highly advanced control such as a steered wheel lock protection at time of braking, a steered wheel slipping protection at time of accelerating and a steered wheel sideslip protection at time of steering.
In the idle stop state, since the engine of the vehicle stops and the vehicle stops and does not run, there is no need of an advanced braking control, and power supply for the VSA is generally shut off. In this state, the VSA obtains a steering angle detected by the steering angle sensor in order for a control of sideslip protection at time of steering, etc., and when power supply for the VSA is shut off, power supply for the steering angle sensor is also shut off. Consequently, every time an idle stop operation is performed, power supply for the steering angle sensor is shut off, so that the midpoint of a steering angle which has been learned is lost, as well.
Meanwhile, in order to keep controllable the steering operation that is a principle operation of a vehicle, power supply for the electric power-assist steering system is maintained even during the idle stop, so as to provide an assist control on the steering operation, not relied on the steering angle. However, although the engine restarts after the idle stop operation, the steering angle sensor cannot detect a steering angle until the steering angle sensor learns a midpoint of the steering angle again, so that the electric power-assist steering system cannot provide a steering assist control based on the steering angle; which may cause an unpleasant steering feeling for a driver.
In order to address the above problem, the present invention has an object to provide an electric power-assist steering system that provides a steering assist control in accordance with the steering angle right after the idle stop operation, thereby to reduce an unpleasant steering feeling for a driver.